The cause of relapse in acute myeloid leukemia (AML) following allogeneic stem cell transplant is unknown, with many theories under investigation. John DiPersio, MD, PhD, of Washington University School of Medicine, Washington, WA, discusses our current understanding of this event from the International Symposium on Acute Leukemias (ISAL) 2019, held in Munich, Germany, with a major focus on CT2A.